Heavy-load service railroads overseas have been increasing train speed and load-carrying capacity of freight cars as a means for improving the efficiency of freight transportation services. Such improvements in efficiency have been attended with severer service environments which, in turn, have needed further improvements in the quality of rails. In such environments, concretely, rails used in curved segments of railroads are rapidly worn down in their gauge corner and the side of their head, and such wear seriously impairs the service life of rails. However, high-strength (or high-hardness) rails of eutectoid carbons steels containing fine pearlite can be prepared by the recently developed strengthening heat treatment technologies described below. Such rails have remarkably lengthened the life of rails used in curved segments of heavy-load service railroads.
(1) A process for manufacturing high-strength steel rails having a strength of 130 kgf/mm.sup.2 minimum by applying accelerated cooling to the head of as-rolled or reheated rails from the austenite region to temperatures between 850 and 500.degree. C. at a rate of 1 to 4.degree. C. per second. (Japanese Patent Publication No. 23244 of 1988)
(2) A process for manufacturing heat-treated low-alloy steel rails having increased wear resistance and improved weldability (permitting easy welding and forming welded joints having good properties) by adding chromium, niobium and other alloying elements. (Japanese Patent Publication No. 19173 of 1984)
These rails are high-strength rails characterized by the presence of fine pearlitic structures obtained in steels containing eutectoid carbon (with a carbon content of 0.7 to 0.8%). The object of these rails is to increase wear resistance by producing a very fine lamellar spacing in pearlite and, at the same time, improve the properties of welded joints by alloy additions.
In straight and gently curved segments of railroads where there does not constitute a serious problem, conventional as-rolled rails of steels with pearlitic structures and some high-strength heat-treated steels have been used. As service environments have grown severer recently, however, repeated contact of rails with train wheels often cause surface fatigue failures in their rolling surfaces. Cracks in the surface of rail heads called "head surface shelling" or "dark spot" are considered particularly important. Cracks of this type occurring in the head surface of rails, propagating to the inner part of their head, and branching to their base sometimes cause transverse fissures in rails for heavy-load services.
It has been known that this dark-spot cracking occurs not only in rails for heavy-load services but also in those for high-speed passenger transportation. The dark-spot cracking is thought to result from the accumulation of fatigue-damaged layers (where pearlite lamellae are ruptured) in the surface of rail heads through the repeated contact of rails with train wheels and the occurrence of slip in the ferrite phase of the pearlitic structure caused by the development of texture (where crystal faces of crystal grains are oriented in the same direction).
This problem can be solved by removing the fatigued layers (fatigue-damaged layer and texture) by grinding off the surface of the rail head. However, grinding that must be done at regular intervals is costly and labor-intensive.
Another solution is to decrease the hardness of the surface of the rail head so that the surface is removed by wear before the fatigued layer is formed. When the hardness of the rail head surface is simply decreased, however, some plastic flow tends to occur in the surface of the rail head directly below the running wheels of the train. The metal flow is oriented in a direction opposite to that of travel of the train running thereover. Then, cracks tend to occur along the metal flow.
The inventors experimentally verified the relationship between the formation of the fatigued layers (fatigue-damaged layer and texture) resulting from the repeated contact of rails with train wheels and the metal structure. The verification study revealed that fatigued layers tend to accumulate and textures tend to develop in pearlitic structures in which ferrite and cementite phases are layered. In bainitic structures in which hard granular carbides are dispersed in the soft matrix of ferritic structures, in contrast, the incidence of accumulation of fatigue-damaged layers and development of textures triggering surface fatigue failures in the metal surface is low, entailing a lower incidence of dark spots.
With heavy-load service railroads overseas, pressures and traction forces at the contact surfaces between rails and wheels are high. Rails made of steels having bainitic structures can prevent dark spots and other fatigue failures in their surface. However, increased wear shorten the service life of rails and increases the incidence of metal flow in the surface of rail heads directly below train wheels. Particularly in gently curved segments where large traction forces are developed, the incidence of other types of fatigue failures in the surface, such as head checks cracks and flaking in gauge corners, increases.
To solve these problems, the inventors sought to devise a method for increasing the strength of bainitic structures. The strength of bainitic steels is governed by the hardness of the ferrite matrix and carbides and the size of carbides in bainitic structures. Generally, the strength of bainitic steels is increased by (1) increasing the hardness of the ferrite matrix and carbides by giving large alloy additions, and (b) reducing the size of carbides by controlling the bainite transformation temperature.
However, large alloy additions required for increasing the hardness of the ferrite matrix and carbides are costly. At the same time, increased hardenability forms martensitic and other structures detrimental to the toughness of rails when they are welded. Although, on the other hand, reduction in the size of carbides increases strength, it is difficult to secure the required wear resistance if the size and quantity of carbides are improper.
By focusing attention on bainitic structures in which fatigued layers (surface fatigue damage and textures) are difficult to form, the inventors sought a method for improving resistances to wear and metal flow without requiring large alloy additions. Specifically, the optimum size for carbides to be achieved by size control was experimentally verified.
It was revealed that when the carbides in bainitic structures are larger than a certain size wear resistance decreases and metal flow causes cracks and other damages. When the carbides in bainitic structures are smaller than a certain size, on the other hand, it is difficult for hard carbides that contributes to the attainment of wear resistance of bainitic steels to accumulate beneath rolling surfaces. Thus, sufficient improvement in wear resistance is difficult to achieve.
In addition to these studies, the inventors also verified the quantity of carbides of the optimum size required for improving resistance to wear and metal flow. This study revealed that when the area occupied by hard carbides of optimum size in a given cross section becomes smaller than a certain limit it is difficult for hard carbides contributing to the attainment of wear resistance of bainitic steels to accumulate beneath rolling surfaces, entailing the lowering of wear resistance. When the quantity of hard carbides of optimum size exceeds a certain limit, on the other hand, ductility of bainitic structures decreases and the incidence of spalling and other flaking failures increases.
Based on these studies, the inventors empirically discovered that bainitic structures having good resistance to surface fatigue failures and wear can be obtained by controlling the size of carbides in bainitic structures and the area occupied by such carbides in a given cross section within certain ranges.
Thus, the object of this invention is to provide high-strength rails having good resistances to surface fatigue failure, wear and metal flow required of heavy-load service railroads at low cost by employing the knowledge obtained by the studies described above.